cmake: Solaris' SunPro has alternative way to link to pthread libraries

Fixes libsdl-org/SDL#10096

(cherry picked from commit 4fbd6cba60)

cmake/sdlchecks.cmake

@@ -963,7 +963,11 @@ macro(CheckPTHREAD)
       set(PTHREAD_LDFLAGS "-lpthread")
     elseif(SOLARIS)
       set(PTHREAD_CFLAGS "-D_REENTRANT")
-      set(PTHREAD_LDFLAGS "-pthread -lposix4")
+      if(CMAKE_C_COMPILER_ID MATCHES "SunPro")
+        set(PTHREAD_LDFLAGS "-mt -lpthread")
+      else()
+        set(PTHREAD_LDFLAGS "-pthread -lposix4")
+      endif()
     elseif(SYSV5)
       set(PTHREAD_CFLAGS "-D_REENTRANT -Kthread")
       set(PTHREAD_LDFLAGS "")